1. Field of the Invention
The present invention relates generally to a power tool, and more particularly to a method of controlling a torque output of an impact power tool, and the control apparatus of the impact power tool.
2. Description of the Related Art
A conventional impact tool, take impact wrench for example, is provided with a motor and a hammer mechanism to make the wrench to repeatedly hammer an object, such as a screw. The output torque of the impact wrench is increasing along with the times of the wrench hammering the screw. The conventional impact wrench outputs a constant torque level only when a user pulls the trigger once so that it can't tighten a screw while the torque is too low, and it will damage the screw while the torque is too high.
An improved impact wrench which may adjust the output torque had been provided to the market. This kind of impact wrench provides several levels of torque by pulse width modulation (PWM). The motor provides different speeds in each level because the duty cycle of the pulse widths which drive the motor are different for each level. The times of the hammer mechanism being activated will increase while the speed of the motor is high so that the torque output in high speed level is greater than in low speed level. However, the speed of the motor decreases with the low battery that makes a difference between the motor's speed and the output torque, and therefore the torque control is not precise. Besides, it will take a long time to turn the screw while the impact wrench is switched to a low torque level. Therefore, the conventional impact wrench still needs to be improved.